Diffraction

We have seen in Chapter 6 that sea waves spread, bending around obstacles and penetrating into inlets. Such bending of waves around corners is a common feature of waves in water. Characteristically, when a wave front comes to a narrow opening or to the entrance of a harbour, it spreads out from this new point of origin in a semicircular manner. Somehow the wave seems to 'forget' that it had a plane wave front before it came to the opening and proceeds to spread sideways, at least in part!

The ripple tank experiment

In a controlled experiment in a water tank we can get a clearer view. Plane waves are sent through a narrow opening (by 'narrow' we mean 'not much larger than the wavelength of

Water waves from narrow slit ripple tank. Courtesy of James Ellis UCD School of Physics, Dublin.

the waves). The emerging waves spread out as did the waves on the beach. If we look closely, we can see the apex of a V made up of two diverging lines along which the waves appear to have been 'rubbed out'. Along these lines the waves interfere destructively, giving regions of calm water. If we had a moving picture we would see that these regions are stationary in space, like the nodes in the one-dimensional case of waves in a string.

8.2.5 Huygens9principle and diffraction

Let us apply Huygens' construction to the waves emerging from a slit.

As we see in Figure 8.4, the spherical wavelets emerging near the edge of the slit have no neighbouring wavelets to maintain a parallel wave front. As a result the envelope of wavelets bends slightly around the edge. The waves spread into the 'geometrical shadow' behind the slit.

We can use Huygens' method to reconstruct the pattern made by water waves emerging from the slit. In the

Figure 8.4 A plane wave passes through a narrow slit.

computer reconstruction of Figure 8.5, we see a striking resemblance to what is seen in the ripple tank experiment.

The V-shaped regions of calm water are clearly visible bordering the expanding wave crests. Most interestingly, we see a secondary and a third region of waves on each side of the central region.